This is a case that I hadn't considered when we discussed bug 1267438. Bug 1267438 introduced the ability to add *one* clip that is scrolled by all of the layer's scroll metadatas, and which is intersected with the layer's other clips. However, we also need to support the scenario where there's more than one scroll frame that don't scroll the layer's contents, but do scroll the layer's scrolled clips.

This testcase shows such a case. It has two elements that have position: absolute; clip: rect(...); set on them, and the inner is scrolled by an additional scroll frame. Both need to apply to the fixed layer. The testcase exposes an additional bug with APZ: We ignore the scroll frame's clip as soon as the scroll frame is active. So to get a feel for the intended behavior, run this testcase with APZ off or in a different browser.

Or maybe I'm confused. We should already have that same problem with background-attachment:fixed inside multiple scroll frames. I need to find out how that works again.

It looks like all scroll meta data clips are always applied to a layer, even for scroll metadatas that don't move a layer. This does what I need.